Alastair Cook reached an elusive and rewarding milestone with his 20th Test hundred on day one of England's Investec Test series against South Africa.

The England opener, who put on 170 with Jonathan Trott (71) after Andrew Strauss made a duck in the first over at the Kia Oval, had gone 16 innings since turning his 19th century into a career-best 294 at Edgbaston last August.

His unbeaten 114 not only served the world number ones well in their 267 for three, as they defend their status against their third-placed visitors, but took Cook level with his Essex mentor and England batting coach Graham Gooch as well as team-mate Kevin Pietersen in his country's list of most prolific centurions.

Cook leaves Len Hutton behind in the teens, but at joint fifth in England's three-figure roll call he still trails Strauss by one and Wally Hammond, Colin Cowdrey and Geoff Boycott by two.

Twice since his near triple-century underpinned the win over India which took England to the top of the world rankings 11 months ago, Cook has faltered on 94.

But he made no mistake this time, pushing an undemonstrative single to cover off leg-spinner Imran Tahir to complete his five-hour hundred in 222 balls - having hit 11 fours and one six.

He was rightly in no hurry to establish England's position of authority, on a dry pitch of even pace which is expected to become significantly tougher to bat on as this match progresses.

Cook missed few opportunities to score either, though, as he and Trott recovered so impressively from the shock of Strauss' departure to only the fourth ball of this three-match series.

England chose to bat first on a cloudy morning - and after a rogue shower delayed the start by 15 minutes and Strauss lasted barely a couple more, the second-wicket pair shut out South Africa's much-hyped attack for more than two sessions.

World number one fast bowler Dale Steyn was off the pitch for treatment to an ankle injury during a much brighter afternoon, but still got through 21 overs to no avail by stumps.

England's day could hardly have started any worse when Strauss was lbw to Morne Morkel, via DRS.

South Africa captain Graeme Smith, as well as Morkel, deserved credit for striking the first blow against his opposite number.

Smith began his 100th Test by promoting Morkel to take the new ball - in place of Steyn, who has shared it with Vernon Philander since the latter began his international career.

It took some courage too, as well as good judgment, to risk a review so early in proceedings - after umpire Steve Davis had turned down Morkel's lbw appeal against the left-hander from round the wicket.

Hawkeye simulated a straightening of the angle and leg-and-middle impact, and England were under significant pressure without a run on the board.

When Strauss had gone for a first-ball duck in his last Test against these opponents, in the innings defeat in Johannesburg two and a half years ago, out-of-sorts Trott played a short and fretful innings he has doubtless been trying to forget ever since.

This time, he drove his first ball calmly past mid-on for four - and a frantic first over concluded with a Steyn misfield in the same position, and two more runs.

Smith held Steyn back for almost an hour, in awkward batting conditions under floodlights.

Cook and Trott stayed patient, but when South Africa dropped short they began to pick up boundaries across the never-ending Oval square.

For good measure, Cook also counted six with mis-hook at Steyn into the stand at long-leg - only the sixth six of his Test career.

When Cook passed his 50 in mid-afternoon, he also brought up his and Trott's seventh century stand together.

Comparisons with their unbroken 339 in the famous draw in Brisbane at the start of England's 2010/11 Ashes series victory were perhaps a little premature, and in the end the world and Test match players of the year made it only just past halfway to that number before Trott edged a drive behind to give Morkel his second wicket.

His typically determined innings had nonetheless lasted 162 balls, and set the stage not just for Cook to continue but Pietersen to up the ante with his range of stroke.

The latter was caught behind, aiming a pull at Jacques Kallis who ended a stand of 81 with the old ball.